Watches including manual control devices intended to replace the usual external control means, such as push-buttons, are already known and are used to control the various functions of a watch like time-setting or starting and stopping a chronograph. Certain of these devices are composed of at least one transparent electrode arranged on the internal face of the watch-glass, the watch comprising a circuit arranged so as to process the signals which are generated when the wearer of the watch places his finger on the glass opposite this electrode.
A device of this type is disclosed in patent CH 607 872. In the arrangement disclosed in this patent, a conducting layer is arranged on the internal face of the watch-glass so as to form a capacitive sensor. This sensor forms, with an independent capacitor, a capacitive divider. An high frequency alternative voltage is applied to this divider by a generator. A detection device detects the voltage between the independent capacitor and the capacitive sensor. This detected voltage varies between two values, according to whether or not a finger is placed on the watch-glass. A comparator compares this detected voltage with a reference voltage and consequently provides a control signal having either a high logical state or a low logical state according to whether the finger is present or not.
A parasitic capacity is always present between the conducting layer arranged on the watch-glass and earth constituted by the case. This capacity is connected in parallel to the capacitive sensor. The potential U.sub.out detected by the comparator may thus be expressed by the relationship: EQU U.sub.out =U.sub.in (C.sub.ref)/(C.sub.p +C.sub.d +C.sub.ref)
where U.sub.in is the voltage applied to the capacitive divider by the generator, C.sub.p is the parasitic capacity, C.sub.d is the capacity of the capacitive sensor and C.sub.ref is the capacity of the independent capacitor.
Such devices have the disadvantage that, in order to provide them with an acceptable resolution, capacities C.sub.d and D.sub.ref must be substantially greater than that of the parasitic capacity, leading to a high consumption of current which notably reduces the life span of the watch battery. Further, the comparator which compares the voltage detected by the capacitive divider to a reference voltage is an analog component which, in order to have the requisite accuracy, must comprise a complex arrangement of transistors.
These disadvantages are particularly felt in the case of a watch including a large number of manual control devices. This is the case, for example, of so called "handwriting recognizing" watches which can receive commands when the user's finger "writes" a sign, for example an alphanumerical sign, such as a figure on the watch-glass. The watch must in such case be equipped with several tens of control devices each having a capacitive sensor arranged on the surface of the glass to enable the position of the finger on the external face of the glass to be recognised instantaneously. The consumption of each control device in such case constitutes a significant factor in determining the life span of the watch battery.